These are what we call the 'YMMV items.' Things that some people find in this work. We call them 'your mileage might vary' because not everyone sees these things in the same way. This starts discussions in the trope lists, a thing we don't want. Please use the discussion page if you'd like to discuss any of these items.
Even though you have eight party members, it is easy to just stick with Goombario most of the time, since he will tell you interesting info about every area and person, reveal the health of enemies, and his basic attack is one point stronger than everyone else. If you equip the badge that lets you switch out a party member without wasting a turn, it is even easier, since you can just retrieve a party member than can defeat an enemy that Goombario cannot, and switch him back afterward.
Watt's basic basic attack ignores enemies' defense, which is really useful against later bosses. She can also attack flying enemies, but does not jump on them, avoiding damage from spiked enemies.
The only thing anyone ever levels up is the Badge Points stat (BP) to get access to as many badges as possible. An NPC can be paid to relocate the extra 5 HP you start with (you start with 10) to BP so that Mario is permanently in the Danger state and can always reap the rewards of many badges that take effect in this state. These badges do things like increase Mario's attack and defence, and even make enemies miss sometimes, and they stack, so you make Mario able to do tons of damage and take absolutely none, effectively turning on god mode. This is known as the "Danger Mario" build.
Demonic Spiders: The Anti Guy with its 50 HP and 10 Attack will slaughter any player that doesn't level up extensively through grinding. If you fail the Quiz portion of Bowser's Castle, you'll have to fight three Anti Guys at once. Unless you're cheating or overpowered, you should reset the game and save yourself from a Curb-Stomp Battle.
The Dark Koopas are very annoying to fight as well, since they have a move that can put Mario into Dizzy-state, making him completely useless for two turns. The problem here is that, more often than not, you fight against multiple Dark Koopas at the same time, and they all like to use that Dizzyfying move on him, giving him absolutely no chance of recovering from it. If you're out of BP when that happens, you might as well reload from your last saved point.
Hammer Bros. in Bowser's Castle. They have a whopping 12 HP, won't flip over like other shelled enemies, and their hammer throws have the possibility of making Mario shrink, cutting his attack power by half. It gets worse when their HP drops below four and can throw a flurry of hammers at him. Their annoyance is even Lampaded by Goombario.
Game Breaker: The Mystery item is a minor one. In battle, it takes the effect of a random item, from a healing Mushroom, to a powerful Thunder Bolt, to a rock that damages Mario. Not so great unless you're in the Random Number God's good graces. Its real value, however, comes from Item Crafting. By giving them over to Tayce T. to cook, you can potentially obtain any craftable item in the game, including ones you're not supposed to get until much later. Most of the time, however, you'll get a Mistake (a Joke Item that only restores 1HP and 1FP apiece), but even that can be utilized by selling them at Boo's Manor for five coins. Conveniently, Boo's Manor sells Mysteries for only 1 coin, giving you a 4 coin profit even in the worst case scenario.
Jerkass Woobie: The Boos who terrorized Tubba Blubba before he gained his powers. However, while Bow admits she's not sorry, she promises that they'll leave him alone after Mario defeats him.
That One Attack: While the Final Boss (Bowser, obviously) isn't overwhelming, he will occasionally use the Star Rod to A) hit both you or your partner (KOing them for three turns) and debuffing both or B) Restore 30 HP.
What an Idiot: A few times. Bombette and the Bob-Ombs in chapter one are trapped in a cage. Said cage has a cracked wall Bombette can easily destroy. She does this immediately after joining you and claims she just didn't think to do it before. Then of course there's Jr. Troopa, who swims between Toad Town and Lavalava Island to chase Mario, and back again. He forgets that he can fly, making him a much easier opponent.